class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int n = s1.size(), m = s2.size();
        if (s3.size() != n + m)
            return false;
        vector<vector<bool>> dp(n + 1, vector<bool>(m + 1, false));
        dp[0][0] = true;
        for (int i = 1; i <= m; i++)
            if (s2[i - 1] == s3[i - 1])
                dp[0][i] = true;
            else
                break;
        for (int i = 1; i <= n; i++)
            if (s1[i - 1] == s3[i - 1])
                dp[i][0] = true;
            else
                break;

        for (int i = 1; i <= n; i++)
        {
            for (int j = 1; j <= m; j++)
            {
                dp[i][j] = (s1[i - 1] == s3[i + j - 1] && dp[i - 1][j]) 
                    || (s2[j - 1] == s3[i + j - 1] && dp[i][j - 1]);
            }
        }
        return dp[n][m];
    }
};